1. Field of the Invention
The present invention relates to the field of thyristors, and more particularly, to the field of metal-oxide-semiconductor (MOS) controlled thyristors which are also known as MCTs.
2. Prior Art
Thyristors are four layer (NPNP) semiconductor devices which latch in a conducting state when their current exceeds a holding current and remain in a conductive state because of internal regenerative action until turned off by removal of the signal applied to their main electrodes or by application of a turn-off signal to an appropriately connected turn-off electrode.
Since thyristors are power devices which are intended to carry large currents, greater than 1 ampere, normally greater than 10 amperes and often more than 100 amperes, substantial effort has been expended in attempting to minimize all resistances between the external package leads in the main current paths of these devices. This effort has been variously directed to reducing contact resistances and the resistances of the portions of the current path which are disposed within the semiconductor body. To this end, the high injection efficiency (heavily doped) portions of the emitter regions are directly connected to the power electrodes via heavily doped portions of the emitter regions to minimize the resistance of the connections between the power electrodes and the injecting portions of the emitter/base junctions. It is also normal practice to minimize the maximum length of the current path between each power electrode and the portion of its associated emitter/base junction which is most remote from the power electrode. This aids in reducing ON-state resistance and in making current flow more uniform. The purpose of this effort has been to minimize the ON-state resistance in order to minimize the ON-state voltage drop and power dissipation in the device.
Early gate turn-off thyristors had a gate electrode in ohmic contact with one of the inner or base layers of the structure. This structure required that the gate draw from one-third to one-half of the ON-state current of the thyristor in order to turn off the thyristor's regenerative action. More recently, MOS controlled thyristors (MCTs) have been developed in which an insulated gate electrode functions as a turn-off gate while drawing miniscule gate current. Such MCTs are beneficial because of the low power levels required in the gate drive circuitry, but can be subject to a problem of conducting more current in their latched state than their MOS gate can turn off with the result that gate control is lost for such high currents.
Consequently, there is a need for an MCT structure which is capable of turning off arbitrarily large currents.